This application relates to carbon nanotubes. More particularly, it relates to structures and their preparation containing 3-dimensional distributions of carbon nanotubes, where the number density of the carbon nanotubes per unit volume far exceeds that previously possibly by prior art methods. Our application specifically relates to the preparation of carbon nanotubes on and within a nonwoven network of small diameter interconnected fibers, including novel general methods of carbon nanotube preparation, the structures resulting therefrom, and some selected properties of these novel structures.
Although graphite and diamond have long been known as structures available to elemental carbon, the preparation of carbon structures in the form of C60 and other fullerenes has sparked intense investigation of even more novel molecular carbon structures. An early discovery was that of carbon nanotubes (S. Iijima, Nature 354, 56 (1991)), which are helical microtubules of graphitic carbon. The simplest carbon nanotubes are single-walled, i.e., a tube formed from a graphitic sheet rolled up on itself with a helical pitch and joined seamlessly at the edges. Usually such tubes are capped at the end to afford a closed tubule with a conical cap. Diameters of 10-20 Angstroms are common.
Multi-walled carbon nanotubes are one step up in complexity and consist of a multiplicity of concentric tubes, each formed by closure of a graphitic sheet, with the distance between concentric tubes being about 0.34 nm, which is the spacing between sheets of graphite. Multi-walled carbon nanotubes may contain only 2 concentric tubes, or may contain 50 or more concentric tubes.
The preparation of carbon nanotubes is possible using diverse methods; see C. Journet and P. Bernier, Appl. Phys. A 67, 1-9 (1998), for a review of preparative methods. Electric arc discharge is perhaps the most widely used technique to produce nanotubes and is based on an electric arc discharge generated between two graphite electrodes in an inert atmosphere such as helium or argon. A plasma, with a temperature on the order of 4000xc2x0 K, is created between two closely spaced electrodes and carbon is sublimed from the anode onto the cathode. Normally only multi-walled carbon nanotubes are formed by this method, but if a metal is introduced as a catalyst (e.g., Co, Ni, Fe, Lu, and combinations thereof) single-walled carbon nanotubes also may be formed.
Laser ablation, the first technique used to generate fullerene clusters in the gas phase, may be used to vaporize graphite in an inert atmosphere. In this method a laser beam is scanned across a heated graphite target area over which flows an inert gas such as helium or argon. Carbon species produced are swept by the gas onto a cooled target, e.g., copper. Where the graphite is heated to 1200xc2x0 C., closed multi-walled carbon nanotubes with 2-24 graphitic layers and a length of up to 300 nm were formed. However, when a small amount of a transition metal is added to graphite, single-walled carbon nanotubes are the predominant product. These tend to organize into rope-like crystallites 5-20 nm in diameter and up to several hundred microns long. An advantage of this method for production of single-walled carbon nanotubes is that very little soot accompanies nanotube formation.
High-temperature gas phase decomposition of hydrocarbons also can be utilized to form carbon nanotubes. For example, nitrogen containing 10% acetylene passed over a catalyst of, e.g., Co or Fe at 500-1100xc2x0 C. results in the formation of multi-walled carbon nanotubes. However, encapsulated metal often is found inside the tubes. One feature when using a support of, e.g., silica or a zeolite, is that the carbon nanotubes formed are not covered by amorphous carbon, as is the case with many other methods. In the work of A. Fonseca and coworkers, Appl. Phys. A 67, 11-22 (1998), ion-exchanged Coxe2x80x94Y zeolite was inactive in the formation of carbon nanotubes, whereas Co-impregnated Y zeolite was quite effective in carbon nanotube production. This showed that carbon nanotubes were formed on the zeolite surface, not in the pores, and thus carbon nanotube deposition (being only at the zeolite surface) can be viewed as having only a two-dimensional distribution. This is an important observation to which we shall return.
Carbon nanotubes have received a great deal of attention in part because of their interesting and sometimes unique properties which make them attractive in many potential applications. Their high strength-to-weight ratio makes carbon nanotubes one of the stiffest materials ever made. Whereas traditional carbon fibers have a strength-to-weight ratio about 40 times that of steel, carbon nanotubes have a strength-to-weight ratio of at least 2 orders of magnitude greater than steel. Carbon nanotubes also show outstanding flexibility and elasticity. Theoretical studies suggest a Young""s modulus as high as 1-5 Tpa, and some measurements have provided an average value of 2 Tpa. Being graphitic, one can expect carbon nanotubes to show high chemical and thermal stability. Recent oxidation studies have shown that the onset of oxidation shifts by about 100xc2x0 C. to higher temperatures in carbon nanotubes compared to graphite fibers. Theoretical considerations predict that carbon nanotubes will show high thermal conductivity in the axial direction.
Whether carbon nanotubes are conductors or semiconductors depends upon the helical pitch of the graphitic sheets forming the nanotube. One of the most important properties of carbon nanotubes in this invention is that of field emission of electrons. Because of the small radius of curvature of the end caps in carbon nanotubes, electrons can be easily extracted into a vacuum by a relatively small external electric field. Since carbon nanotubes may be deposited on large substrates, this makes them suitable for applications such as flat panel displays. Compared with other carbon-based cold cathode electron field emission materials, carbon nanotubes have relatively low turn-on electric field and high current density, arising mainly from the high field enhancement factor introduced by their very high aspect ratio and small curvature.
In cold field emission applications, it should be clear that it is highly desirable to obtain maximum electron flux from a given material Where carbon nanotubes are the emission source, it thus follows that what is required are structures having the largest possible number of nanotubes per unit cathode surface area that actually emit electrons. We have previously noted that although prior methods produce large numbers of carbon nanotube deposition in a 2-dimensional distribution, i.e., on the surface of whatever material is used as a substrate, only a very small faction of the carbon nanotubes contribute to electron field emission. Only those carbon nanotubes that are taller than others and therefore at a smaller distance to the anode are subjected to the higher electric field necessary for electron field emission. When the applied voltage is increased in order to raise the local electric field at the tips of those shorter carbon nanotubes, the electric field becomes too high for taller carbon nanotubes causing them to be damaged by overheating because too high a current is forced to flow through too few carbon nanotubes. Using a porous substrate such as silica or a zeolite provides no exception; nanotube formation is limited to the substrate surface and fails to occur within the substrate pores. Clearly, then, what is needed is a substrate which provides the macroporosity requisite for a 3-dimensional deposition of carbon nanotubes. The resulting material has a sponge-like structure with carbon nanotubes distributed throughout the material, rather than solely on the surface. By suppressing the growth of carbon nanotube on the surface of the substrate and allowing carbon nanotubes to grow inside the pores of the macroporous substrate or by removing carbon nanotubes that stick out of the macroporous substrate surface, only carbon nanotubes residing below the macroscopic substrate surface are contributing to the electron emission. This method minimizes the burnout of longer or taller carbon nanotubes as it would occur in the case when carbon nanotubes are allowed to grow only on the surface of the cathode as explained previously. As a result, the number of electrons emitted per unit macroscopic cathode surface area is maximized. Our invention affords just such a sponge-like material using a broad class of substrates with varying properties.
Another important property of the sponge-like carbon nanotube structure described in this invention is the extremely large effective surface area per unit volume as well as per unit weight. This property is highly desirable for electrode applications that require as large an electrode surface area as possible. The sponge-like carbon nanotube can serve as highly effective electrodes for a super-capacitor, a high energy density battery, and a high density catalyst support applications, to name a few.
In particular, our invention uses a 3-dimensional nonwoven network of interconnected randomly oriented fibers as a substrate. Such a substrate has high macroporosity arising from the interstices between the fibers and affords many places where carbon nanotubes can grow. Additionally, the nanotubes grow on the fiber surfaces, generally perpendicular to, but sometimes at an angle to, the fiber axis, so that there results a high density of formed carbon nanotubes per unit volume. We also have provided several methods of producing such carbon nanotubes on our substrate so as to provide several optional preparative methods, each with their own advantages. These preparative methods, even though particularly valuable to production of our sponge-like materials, have general applicability to carbon nanotube formation and may be advantageously utilized with substrates different from ours.